1. Field of the Invention
The present invention is related to an optical disc used for a high-density recording, particularly, related to an optical disc from which recording signals are reproduced by irradiating a reproducing light on a recording surface thereof through a resin film and adhesive layer formed in the optical disc.
2. Description of the Related Art
Recently, in the field of information recording, the study for an optical information recording method has been forwarded in various places. This method has such various advantages as to record or reproduce information signals in a non-contact manner, to attain a high recording density more than 10 times compared with the magnetic recording method, and to provide various memories such as a read only memory, a rewritable (or writable) memory and a write once memory. Thus, this optical information recording method is considered to be one of methods capable of realizing a large-capacity file in low cost, and its wide application, from industrial use to consumer use, has been studied in various fields. Among all, a digital audio disc and an optical videodisc known as an optical disc corresponding to the read only memory have spread over the world.
In the above optical disc such as an audio digital optical disc, a reflecting layer of a metal such as aluminum is formed on a transparent substrate (or an optical disc substrate) on which a group of pits and grooves representing information signals are formed in a concavity or convexity pattern. Further, a protecting layer is formed on the reflecting layer for preventing erosion and flaws from developing on the reflecting layer.
Upon reproducing information signal from the optical disc, the reproducing light such as a leaser beam is irradiated on the concavity or convexity pattern from a side of the optical disc substrate, and the information signal is detected as a difference of reflectivity caused by the concavity or convexity pattern. In other words, it is detected as a difference of a light intensity between an incident light and its reflected light from the concavity or convexity pattern.
Upon manufacturing such an optical disc, the optical disc substrate is formed to have the concavity or convexity pattern by using such an injection molding method. Then, the reflecting layer of such a metal as mentioned above is formed thereon. Further, the protecting layer made of, for instance, an ultraviolet curing resin is formed thereon.
Recently, a further high-density recording has been demanded. In order to respond to this demand, there is proposed a countermeasure in which a numerical aperture (referred to as NA hereinafter) of an objective lens installed in an optical pickup for irradiating the reproducing light is made to be larger to reduce a radius of a spot of the reproducing light. For instance, the NA of the objective lens used for the digital audio disc was 0.45 so far.
On the other hand, in the optical video disc (for instance, Digital Versatile Disc, referred to as DVD, hereinafter), which has a large recording capacity of more than 6-8 times compared with that of the digital audio disc, the NA of the objective lens used for the optical video disc is made to be about 0.60.
As the NA of the objective lens is made to be larger, it is necessary to reduce a thickness of the optical disc substrate through which an incident reproducing light passes. This reason is that the thicker, a thickness of the optical disc substrate becomes, the fewer, an allowance for a tilt angle to an optical axis of the optical pickup becomes, and when the tilt angle becomes larger, the reflected light is badly affected by aberrations and birefringence caused by the thickness of the optical disc substrate. Here, the tilt angle to the optical axis of the optical pickup is defined as an angle of an optical axis thereof deviated from a normal line to a surface of the optical disc. Thus, the thickness of the substrate is made to be as thin as possible to prevent the bad effect of aberrations and birefringence. For instance, in the digital audio disc mentioned in the foregoing, the thickness of the substrate is made to be 1.2 mm. On the other hand, in the optical videodisc such as the DVD, which has a recording capacity 6-8 times as large as the digital audio disc, the thickness of the substrate is made to be about 0.6 mm.
However, it seems that the thickness of the substrate needs to be made much thinner to satisfy a demand for a further high-recording density.
As a countermeasure thereof, there is proposed an optical recording medium having a light transmission layer having a thin thickness on the reflecting layer. Specifically, an information recording layer with cavities, so called pits, is formed on a main surface of the optical disc substrate, and there are formed a reflecting layer thereon. Further, the light transmission layer for allowing a light to pass through is formed on the reflecting layer. Upon reproducing, the reproducing light is irradiated from the side of the light transmission layer. Thereby, it is possible to provide the benefits of a large NA of the objective lens by thinning the thickness of the light transmission layer.
When the light transmission layer is made to be thin, however, it is difficult to successfully manufacture such a thin light transmission layer having a thickness of 0.1 mm with a thermoplastic resin by using an injection molding machine without degrading the birefringence and transparency thereof in the prior art.
As a countermeasure thereof, there is proposed a method for manufacturing the light transmission layer with an ultraviolet curing resin in the Japanese Patent Laid-open Publication No. 8-235638/96. However, there is a problem that it is difficult to manufacture the light transmission layer having a uniform thickness, resulting in a difficulty to reproduce the information signals stably.
As a countermeasure thereof, there is proposed another method for providing the light transmission layer on the reflecting layer in the Japanese Patent Laid-open Publication No. 10-283683/98, wherein a resin film having a thickness of 0.1 mm is bonded on the reflecting layer as the light transmission layer with an adhesive by a roll-pressure method or with an ultraviolet ray curing adhesive by a spin coating method.
Incidentally, as one of the important characters for the optical disc, there is a birefringence phenomenon on the light transmission layer for allowing the light to pass through. The birefringence implies a phenomenon that an incident light impinging on an anisotropy material is separated into two lights vibrating in a perpendicular direction to each other. These two lights propagate in a different velocity, respectively, resulting in an optical path difference (a phase difference) when they have passed through the anisotropy material. This optical path difference has to be made as small as possible because it badly affects the reproducing signal characteristics. As conventional countermeasures thereof, the injection molding technique and the materials for the disc substrate have been improved ever since. These improvements have been made in a composite type optical disc as well.
On one hand, in the case of the optical disc where the light transmission layer is formed by bonding the thin film made of resin on the reflecting layer with the adhesive, there is a problem that the birefringence of the thin film is increased in a process of bonding or upon reproducing information signals at a high speed rotation of the optical disc due to a centrifugal force.
Specifically, in the case of providing the thin film on the reflecting layer by using the roll pressure method, a stress-strain is partially developed in the thin film because of deviations of pressure given from the roller and of thickness of the thin film. Thereby, the birefringence is increased to badly affect the reproducing signal characteristics of the optical disc, especially to such an extent that it badly effects a deviation of the reproducing signal output occurring during one rotation of the optical disc.
On the other hand, in the case of the optical disc where the thin film is formed with the ultraviolet ray curing adhesive by using the spin coating method, there is a problem that a curing strain is developed in the adhesive layer when the curing speed is promoted by increasing an intensity of ultraviolet ray, or the adhesive layer is cured in such a state that the adhesive layer involves dusts and foams in the bonding process.
Further, upon reproducing the information signal from the optical disc, the optical disc is rotated at a high speed. Thus, a strain is developed in the adhesive layer because of the centrifugal force thereof.
As mentioned above, when the strain is developed in the adhesive layer, the thin film is also strained because they are directly joined, resulting in an increase of the birefringence in the thin film. Further, upon using the optical disc having such a strain in the adhesive layer and the thin film under a humid and hot circumstance (for instance, in a car in a midsummer), the thin film is softened because it is made of a resin. Thus, the strain therein increases, resulting in an increase of the birefringence thereof. This poses a problem that the reliability of the optical disc is degraded.